what's wrong with my enlarger?

ok, I am a beginner in darkroom printing and the other night I noticed that I can't get a print sharp from corner to corner of my paper.
and it's not an issue regarding the image on the neg, I can't focus on the grain on the whole picture. to me looks like the neg is not held perfectly parallel with the paper plane.
apologies for not being technical, but as I said, I am a beginner
and if helps, I use a Meopta Axomat with colour head and a Rodenstock Rodagon stopped down to f8 or 5.6...

First try to align your enlarger with a spirit-level , every thing should be level !
Start at the board and work your way up: lensplate, neg carier.
Check the alignment of your neg carier aswell, look at it from the side, everything should be parallel to each other.

Check if the Rodagon is mounted correctly, it is a good and sharp lens.

I might add some more information in order to make your quest easier.
I'm dealing with hp5 35mm. and I can't get the focus right for the whole picture at the same moment.
it helps if I push down the neg holder, but this is not a valid technique when it comes to printing
will check the alignment tonight, thanks!

When the Neg carrier is in the Enlarger it should nip (not crush) the negative firmly - so it cannot be easily pulled through.

Make sure yours is nipping the Neg tightly enough

Some enlargers have a slight detent to hold the carrier off being very tight - to allow you to slide the next Neg on the strip into place without removing the whole carrier - so make sure yours is closing properly.

Also, it cant hurt to set the apeture higher than f8 may well improve the flat field of focus

Well, this is one way to solve it. But going to a smaller f stop will cut down on sharpness. Aligning the enlarger is the way to go. Most lenses are at their sharpest one stop from wide open. If the enlarger is aligned there isn't much reason to stop down more, except maybe for light control.

thanks everybody for the help and support. the neg holder seems to work fine. keeps the neg in place and it's resonably tight.
the only thing that bothers me it's the fact that it's not solid when slided in the head. actually I can tilt it with a little pressure.
if you're interested about the head alignment, here's a picture I took. as a reference, there's a hotshoe spirit level.

@phenix: I can fit just 50mm lens and I am pretty sure that the lens board (that threaded 'dish' where the lens screws into, if I understood correctly) is mounted properly..